From your question I think you do NOT plan to save ANY of the information already on the hard drive. Instead you plan to re-install Win 7, using the Install CD and installing to the hard drive already in the machine. I am going to assume you may have data on the HDD that you will want to get rid of so it is empty for the fresh installation.
Put your Install CD in your optical drive and reboot the computer. As you do this, hold down the "Del" key to make your machine go into the BIOS Setup screens and NOT boot from your hard disk. Watch the screen carefully. The "Del" key is a common way to do this, but you might see a message that tells you to use a different key to enter BIOS Setup.
As the POST starts up some messages will show up, and you keep holding down the "Del" key until it pops into the opening Setup screen. There will be prompts to the right and at the bottom how to move around the Setup menus. You will need to find the place where you specify the Boot Priority Sequence, probably in Advanced or Peripheral Devices. Set this to try the optical drive as the first boot device, and your hard drive as the second, and NO other devices after that. Use the correct key to Save and Exit, and your machine will save your new setting and reboot, this time from the Install disk already in the optical drive. That will start the new Install process.
As a first step for the fresh installation, I suggest you look at the menu choices and find a place where you can Delete the Partition(s) that already exist on your hard drive. LAST CHANCE! This WILL destroy any old data on the drive - do your really want to do that? Assuming you do, make sure you Delete all the Partitions it finds there. Then you can proceed with the normal Install process onto a completely empty drive.
The rest is the same as if you were just starting an Install on a brand new machine.